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      SpringBoot系列—@SpringBootApplication源码解析（三）
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        <p>&emsp;&emsp;上一篇，内容很少，说了一下SpringBoot的底层是怎么依赖的。这一篇讲一下SpringBoot底层源码是怎么实现的。</p>
<a id="more"></a>
<p>&emsp;&emsp;从SpringBoot的入口开始。第一篇的时候说过了，最好把入口文件放在最外层的包下面，至于原因就不多阐述了。</p>
<p>&emsp;&emsp;说说这个 <strong>@SpringBootApplication</strong>注解的原理,点进去看一下：</p>
<p>如下图：</p>
<p><img src="/uploads/article/2019-08-22/springbootapplication注解.png" alt="springbootapplication注解.png"></p>
<p>主要就是这三个注解：</p>
<ul>
<li>@SpringBootConfiguration</li>
<li>@EnableAutoConfiguration</li>
<li>@ComponentScan</li>
</ul>
<h3 id="SpringBootConfiguration"><a href="#SpringBootConfiguration" class="headerlink" title="@SpringBootConfiguration"></a>@SpringBootConfiguration</h3><p>&emsp;&emsp;SpringBootConfiguration这个注解是由Configuration注解实现的。</p>
<h4 id="Configuration"><a href="#Configuration" class="headerlink" title="@Configuration"></a>@Configuration</h4><p>&emsp;&emsp;从名字上就可以判断出，这个注解是跟SpringBoot配置相关的，<strong>@Configuration</strong>，这个注解是Spring底层的一个注解，之前都是配置xml，SpringBoot推荐我们用配置类来描述配置，配置类是什么呢？就用@Configuration标记的类。</p>
<p>&emsp;&emsp;Configuration这个注解是由Component注解实现。</p>
<h5 id="Component"><a href="#Component" class="headerlink" title="@Component"></a>@Component</h5><p>&emsp;&emsp;这个注解的意义就是，把当前的配置类添加到spring容器中，表示是一个组件。</p>
<h3 id="EnableAutoConfiguration"><a href="#EnableAutoConfiguration" class="headerlink" title="@EnableAutoConfiguration"></a>@EnableAutoConfiguration</h3><p>&emsp;&emsp;EnableAutoConfiguration这个注解是由<strong>AutoConfigurationPackage</strong>和<strong>@Import({AutoConfigurationImportSelector.class})</strong>注解实现</p>
<h4 id="AutoConfigurationPackage"><a href="#AutoConfigurationPackage" class="headerlink" title="@AutoConfigurationPackage"></a>@AutoConfigurationPackage</h4><p>&emsp;&emsp;这个注解从名字上面看出来是自动配置包，这是什么意思呢？在第一篇我们就说过了入口要放在最外层的包，至于原因，已经讲过了，因为SpringBoot会自动将引导类 <strong>@SpringBootApplication标注的类</strong>所在的包以及下面所有子包里面所有的组件扫描到Spring容器中统一管理。就是这个注解实现的。那么，这个注解是怎么实现的呢？就是通过它的这个 <strong>@Import({Registrar.class})</strong>，下面就是Registrar类的源码了</p>
<div class="highlight-box"autocomplete="off" autocorrect="off" autocapitalize="off" spellcheck="false" contenteditable="true"data-rel="PLAIN"><figure class="iseeu highlight /plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line">static class Registrar implements ImportBeanDefinitionRegistrar, DeterminableImports &#123;</span><br><span class="line">        Registrar() &#123;</span><br><span class="line">        &#125;</span><br><span class="line"></span><br><span class="line">        public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) &#123;</span><br><span class="line">            AutoConfigurationPackages.register(registry, (new AutoConfigurationPackages.PackageImport(metadata)).getPackageName());</span><br><span class="line">        &#125;</span><br><span class="line"></span><br><span class="line">        public Set&lt;Object&gt; determineImports(AnnotationMetadata metadata) &#123;</span><br><span class="line">            return Collections.singleton(new AutoConfigurationPackages.PackageImport(metadata));</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br></pre></td></tr></table></figure></div>
<p>&emsp;&emsp;就是通过下面这个代码注册进去的。</p>
<div class="highlight-box"autocomplete="off" autocorrect="off" autocapitalize="off" spellcheck="false" contenteditable="true"data-rel="PLAIN"><figure class="iseeu highlight /plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">AutoConfigurationPackages.register(registry, (new AutoConfigurationPackages.PackageImport(metadata)).getPackageName());</span><br></pre></td></tr></table></figure></div>
<h4 id="Import-AutoConfigurationImportSelector-class"><a href="#Import-AutoConfigurationImportSelector-class" class="headerlink" title="@Import({AutoConfigurationImportSelector.class})"></a>@Import({AutoConfigurationImportSelector.class})</h4><p>&emsp;&emsp;这个注解是干什么用的呢？</p>
<ol>
<li>它会把项目里面所有需要导入的组件以全类名的方式返回，将这些组件添加到容器中。</li>
<li>会给容器中注入非常多的自动配置类，就是导入并配置好当前项目中<br>所需要的组件,省去我们手动编写配置去注入组件。</li>
</ol>
<p>&emsp;&emsp;怎么实现的呢？打开AutoConfigurationImportSelector类，找到如下代码：</p>
<div class="highlight-box"autocomplete="off" autocorrect="off" autocapitalize="off" spellcheck="false" contenteditable="true"data-rel="PLAIN"><figure class="iseeu highlight /plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">List&lt;String&gt; configurations = this.getCandidateConfigurations(annotationMetadata, attributes);</span><br></pre></td></tr></table></figure></div>
<p>找到getCandidateConfigurations方法，如下：</p>
<div class="highlight-box"autocomplete="off" autocorrect="off" autocapitalize="off" spellcheck="false" contenteditable="true"data-rel="PLAIN"><figure class="iseeu highlight /plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">protected List&lt;String&gt; getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) &#123;</span><br><span class="line">        List&lt;String&gt; configurations = SpringFactoriesLoader.loadFactoryNames(this.getSpringFactoriesLoaderFactoryClass(), this.getBeanClassLoader());</span><br><span class="line">        Assert.notEmpty(configurations, &quot;No auto configuration classes found in META-INF/spring.factories. If you are using a custom packaging, make sure that file is correct.&quot;);</span><br><span class="line">        return configurations;</span><br><span class="line">    &#125;</span><br></pre></td></tr></table></figure></div>
<p>找到loadFactoryNames方法，如下：</p>
<div class="highlight-box"autocomplete="off" autocorrect="off" autocapitalize="off" spellcheck="false" contenteditable="true"data-rel="PLAIN"><figure class="iseeu highlight /plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">public static List&lt;String&gt; loadFactoryNames(Class&lt;?&gt; factoryClass, @Nullable ClassLoader classLoader) &#123;</span><br><span class="line">        String factoryClassName = factoryClass.getName();</span><br><span class="line">        return (List)loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());</span><br><span class="line">    &#125;</span><br></pre></td></tr></table></figure></div>
<p>找到loadSpringFactories方法，里面有</p>
<div class="highlight-box"autocomplete="off" autocorrect="off" autocapitalize="off" spellcheck="false" contenteditable="true"data-rel="PLAIN"><figure class="iseeu highlight /plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">Enumeration&lt;URL&gt; urls = classLoader != null ? classLoader.getResources(&quot;META-INF/spring.factories&quot;) : ClassLoader.getSystemResources(&quot;META-INF/spring.factories&quot;);</span><br></pre></td></tr></table></figure></div>
<p>这里就是我们的配置，我们这个配置具体是在哪呢？如下图</p>
<p><img src="/uploads/article/2019-08-22/autoConfigure.png" alt="autoConfigure.png"></p>
<h3 id="ComponentScan"><a href="#ComponentScan" class="headerlink" title="@ComponentScan"></a>@ComponentScan</h3><p>&emsp;&emsp;这个注解的主要作用就是，被该注解标识的类会被Spring容器纳入管理。</p>

      
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